Erosional control on the structural evolution of a transpressional thrust complex on the Alpine fault, New Zealand

Abstract

The Waikukupa thrust is a 4-km long oblique thrust segment of the New Zealand Alpine Fault that has developed over the last 65,000 years, emplacing mylonite and cataclasite over fluvioglacial gravel. During the last 20,000 years or so, a 2-km long ‘out-of-sequence’ imbricate, the Hare Mare thrust, has formed on the east side of the Waikukupa River valley. The average rate of slip on the two thrusts is estimated at 22–30 mm/year. Analysis of the fault structure in terms of critical wedge theory is consistent with imbrication resulting from a reduction in wedge taper below the critical value due to rapid river erosion. The internal structure of the Waikukupa thrust sheet consists of reverse faults and angular folds forming duplex-like systems, associated with minor strike-slip faults. Within the limits of uncertainty, the structures are consistent with strain accumulating during internal transpressional shear of the thrust sheet while the taper of the latter was subcritical prior to the transfer of fault displacement to the Hare Mare imbricate. The thrust complex forms part of a serrated range front fault system consisting of oblique thrust sections linked by strike-slip faults. We introduce the term ‘serial partitioning’ for this type of partitioned transpressional fault system. Its development is linked to the erosional processes at the range front. An alternative system consisting of parallel thrust and strike-slip faults we here term ‘parallel partitioning’. While angle of obliquity is an important parameter in determining which partitioning model develops, we suggest that erosion rate is also an influential factor.